Document acceptors, such as those used in the vending and gaming industries, typically contain sensing means to detect the physical presence of a document (e.g., a bill) being processed, or to detect the transitional state of movable elements in the machine. An effective and widely-used type of sensing means is an optical sensing means, which may include a light source and a light receiver. Such sensors typically have no moving parts and do not require physical contact with the object being sensed in order to function properly.
Such sensors sometimes generate signals corresponding to the light transmitted through a document as it moves past. Those signals are processed to determine certain information, such as the position of the document in the passageway and the authenticity of the document. To make such determinations, the signals are compared to data stored in memory that corresponds to genuine bills.